As delineated in the parent application, resonators are traditionally employed as components, in among other things, filters and oscillators. Resonators have gained greater importance lately with the growth of mobile communications technology and the increasing clock speed of computers. Mobile devices require small precision filters, and computer clocks require oscillators capable of error-free high frequency oscillation. Typical resonator applications require resonators to demonstrate Q values higher than one thousand at an impedance load of approximately 50 Ω, be compatible with common voltage ranges of typical integrated circuits, and to resonate at frequencies near or above 1 GHz.
Conventional resonators include, for example, surface acoustic wave (SAW) resonators, fundamental mode thin film resonators (TFRs), flexural mode MEMS beam resonators, guided electromagnetic wave structures, lumped element inductors and capacitors, thin film bulk acoustic resonators (TFBARs), overmoded bulk crystals, and solidly mounted resonators (SMRs). These filters suffer from a variety of shortcomings. For example, many are too large for placement onto an integrated circuit, operate in an unsatisfactorily small frequency range, require too high a voltage for operation, and/or cannot achieve a sufficiently high Quality (Q) level with a 50 Ω load. Q is a measure of the energy efficiency of a filter and also is a measure of the sharpness of the filter's frequency response, i.e., a high Q filter passes a narrower band of frequencies than a lower Q filter.
Many traditional resonators are too large to incorporate one or more resonators onto an integrated circuit. For example, thin-film resonators (TFRs) commonly have a footprint on the order of hundreds of microns. Surface acoustic wave (SAW) resonators are typically even larger, in some cases requiring substrates as large several centimeters or inches to exhibit desirable performance characteristics.
Several traditional resonators operate in an undesirably limited frequency range. MEMS flexural mode beam resonators typically do not operate satisfactorily in the Ultra-High Frequency range. Similarly, TFBARs and SMRs are difficult to make with center frequencies much below 1 GHz since film stress becomes an issue as the film thickness is increased. Furthermore, with most film based resonators, multiple resonators with differing frequencies cannot be placed on a single integrated circuit because the film thickness is typically uniform across a substrate.
Many MEMS flexural mode beam resonators also suffer from requiring activation voltages that make the resonators difficult to integrate in standard integrated circuits. MEMS flexural mode beam resonators are commonly actuated capacitively, in some cases requiring as much a 50V to achieve resonance.
Many conventional resonators do not exhibit high enough Q levels with a 50 Ω load. Typical lumped element inductors, SMRs, and many TFBARs fail to meet the desired Q level of 1000 in response to operating with a 50 Ω load. MEMS flexural mode beam resonators can operate with higher Q levels, but usually require operation in a vacuum to do so.
The parent application Ser. No. 10/631,695 published Feb. 12, 2004, is hereby incorporated herein by this reference.
Capacitively actuated MEMS devices have also been mechanically coupled to achieve lower impedance and/or to introduce multiple closely spaced modes for a filter passband. See M. Demirci et al., Mechanically Corner-Coupled Square Microresonator Array for Reduced Series Motional Resistance, The 12th International Conference on Solid State Sensors, Actuators, and Microsystems, Jun. 8-12, 2003, pp. 955-958, also incorporated herein by this reference. Inherent in the capacitive actuation is high impedance (>104 Ω) unless nano-scale fabrication is achieved. In addition, the device response is linear only over a limited range and high bias voltages may be necessary for operation. Other MEMS resonators can also have multi-tap designs. Those skilled in the art have explored multi-tap designs extensively. For example, surface acoustic wave (SAW) devices have the option of using multiple electrical taps on a single mechanical device as discussed in V. Plessky et al., Balanced Lattice Filter With Acoustically Interacting Resonators, 2002 IEEE Ultrasonics Symposium, pp. 143-145 and V. B. Chvets, Design of Side Band Transversely Coupled Resonator Filters on Quartz, 2002 IEEE Ultrasonics Symposium, pp. 173-177. Both of these papers are incorporated herein by this reference. Thus, there is an opportunity to make a two-port device or a multi-terminal device that has certain advantages in circuit designs. The disadvantage of using SAW devices in general, however, is their large size and limited quality factor (Q). RF Monolithics, Inc., for example, offers a 916.5 MHz SAW filter with a maximum insertion loss of 2.5 dB. The unloaded Q values are given as 23,509 and the 50-Ω loaded Q value is 4,000 (part No. R02144A). However, the footprint of single SAW resonators is on the order of millimeters and they are not readily integrated with other process technologies.
Thin film resonators (TFR's and FBAR's) and solidly mounted resonators (SMR's) are another category of piezoelectric resonators. In these devices, a piezoelectric material is sandwiched by two electrodes in a capacitor-like geometry. The targeted resonance is a compressional wave in the thickness direction rather than in the length or width direction. These devices can also be suspended, so that the acoustic mode does not couple energy into the substrate. The primary differences between TFR's, FBAR's, and SMR's are related to the mechanical mounting and boundary conditions of the device. Mechanical coupling between two or more such resonators can be achieved by stacking them on top of each other as discussed in K. M. Lakin et al., High Performance Stacked Crystal Filters for GPS and Wide Bandwidth Applications, 2001 IEEE Ultrasonics Symposium, pp. 833-838, incorporated herein by this reference. For example, the layers would be: metal, piezoelectric, metal, piezoelectric, and metal. When the shared metal electrode is grounded and the other two electrodes are used as input and output electrodes, this is called the stacked crystal filter (SCF). These devices may be difficult to manufacture, since the stress in the stack must be well-controlled and access to inner metal electrode layers is necessary. Other drawbacks of these devices include the relatively large footprint and the difficulty of fabricating devices with different resonant frequencies, for example film thicknesses on a single chip.
References is also made to S. Diamantis et al., A Programmable MEMS Bandpass Filter, Proceedings of the 43rd IEEE Midwest Symposium on Circuits and Systems, Lansing Mich., Aug. 8-11, 2000 pp. 522-525, incorporated herein by this reference. This paper refers to a flexural mode resonator which is capacitively activated.